Method of exploding or igniting materials using adiabatic compression of gas

ABSTRACT

A method for exploding or igniting a pumpable material having a threshold explosion or ignition temperature which entails mixing with the material, small, discrete quantities of gas; then transmitting a high energy shock wave from a remote location through a coupling agent to the mixture to substantially adiabatically compress the gas to increase the temperature thereof above the threshold explosion or ignition temperature, thereby initiating thermal detonation or ignition of the material to be exploded or ignited. The method is well adapted for use in the stimulation of the production of fluids from subterranean formations.

United States Patent Inventor William L. Hill Ponca City, Okla.

Appl. No. 784,263

Filed Dec. 16, 1968 Patented Aug. 31, 1971 Assignee Continental Oil Company Ponca City, Okla.

METHOD OF EXPLODING OR IGNITING MATERIALS USING ADIABATIC COMPRESSION OF GAS 6 Claims, No Drawings [1.8. CI 166/299, 102/21.6, 102/23 Int. Cl ..E2lb43/26, F42d 1/04, F42d 3/04 Field 01 Search 166/299; 89/7; 102/20, 21, 21.6, 22, 23

References Cited UNlTED STATES PATENTS 2,671,400 3/1954 Duesing 102/20 UX 5/1955 Nowak 166/299 2,867,172 1/1959 Hradel 102/20 UX 2,872,846 2/1959 Crozier 89/7 2,882,796 4/1959 Clark et a1. 89/7 2,947,221 8/1960 Griffin et al. 89/7 3,075,463 l/1963 Eilers et al. 166/299 3,186,304 6/1965 Biehl 39/7 3,456,589 7/1969 Thomison et a1. 102/23 Primary Examiner-Ian A. Calvert Attorneys-Joseph C. Kotarski, Henry H. Huth, Jerry B.

Peterson, Robert B. Coleman, Jr., Carroll, Palmer and Kemon and Palmer and Estabrook ABSTRACT: A method for exploding or igniting a pumpable material having a threshold explosion or ignition temperature which entails mixing with the material, small, discrete quantities of gas; then transmitting a high energy shock wave from a remote location through a coupling agent to the mixture to substantially adiabatically compress the gas to increase the temperature thereof above the threshold explosion or ignition temperature, thereby initiating thermal detonation or ignition of the material to be exploded or ignited. The method is well adapted for use in the stimulation of the production of fluids from subterranean formations.

BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to methods of exploding or igniting materials from a remote location by indirect contact, and more particularly, but not by way of limitation, to a method of exploding an explosive system located in an induced fracture in an oil producing subterranean formation.

2. Brief Description of the Prior Art The use of an explosive to stimulate production ofv hydrocarbons from subterranean formations has long been known, and variousmethods and systems have been proposed for detonating or igniting the explosive. Prior to effecting the explosion, the customary practice is to position the explosive system in an induced fracture extending outwardly from the well bore into the adjacent subterranean formation which carries the fluids to be produced. Various methods have been proposed for then detonating the explosive. In some instances, a detonator device has been lowered in the well bore and moved into or adjacent the fracture so that physical contact is established between this device and the explosive. Positioning of the detonator device in this manner is frequently time consuming, difficult and expensive.

Another method of detonation sometimes employed is that of effecting a chemical reaction between certain chemicals added to the explosive system, with sufiicient heat or shock being developed by such reaction to detonate the explosive. This is sometimes an expensive method of detonation, and placement of the necessary chemicals in proper contact with the explosive is often time-consuming and difficult. Moreover, the timing of the explosion effected in this manner is often more difficult to control than when detonating devices are used.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present invention is a method of igniting or exploding a pumpable material from a remote location by a shock wave transmitted to the material from such location, such method comprising mixing discrete quantities of a gas with the explosive material or system; then transmitting a high energy shock wave to and through the mixture to compress the gas substantially adiabatically and thereby elevate its temperature above a threshold temperature at which detonation or ignition of the explosive system occurs. The method contemplates the inclusion of the discrete quantities of gas in the mixture by encapsulating the gas in small, hollow, glass, plastic or gelatinous films or bubbles, or by entrapping bubbles of the gas in a gelled fluid or emulsion. Preferably, the shock wave is capable of imparting an energy level of at least about 330 grams-cm. at the locus of the gas in order to raise the temperature thereof to a selected value.

It is an object of the present invention to provide an improved method for exploding or igniting a material.

A more specific object of the invention is to provide a method for exploding an explosive from a remote location and without the use of any ignition or detonation device in direct contact with the explosive.

Another object is to provide a method of explosively stimulating production of hydrocarbons from subterranean formations by igniting an explosive located in an induced fracture without the use of an igniter device in contact with the explosive.

Additional objects and advantages of the invention will become apparent from the following detailed description of the invention.

In carrying out the process of the invention, it is desirable to initially mix a gas with the explosive system in such a way that there are small, discrete quantities of the gas located throughout the mixture. The process is operative, however, with but a single, small isolated quantity of gas, such as a large bubble, located in the explosive material at a location susceptible to forces of compression.

The process of the invention depends upon the realization of three conditions. First, the gas must be susceptible to adiabatic compression in the explosive system, so that in undergoing such compression, its temperature will be elevated to create a hot spot." As a second condition, the explosive system must contain a component which is either ignited or detonated at temperatures at least as low as the temperature of the hot spot developed upon compression of gas, and such component must be sufficiently close to the locus of one or more of the hot spots to undergo thermal ignition or detonation when the gas is compressed. Finally, the method contemplates the development or generation of a high energy shock wave which is then transmitted from a remote location through a coupling material to the gas to effect the necessary compression.

Substantially any gas which is compatible with the particular explosive system in use may be used in the practice of the invention, although some gases are better suited than others. Those gases which have relatively high value of 'y, as this term is hereinafter defined, are preferred for use in the process of the invention, since less energy is required in order to compress such gases sufficiently to realize a given temperature level. The 7 value of a gas is defined as the ratio of its heat capacity at constant pressure, C to its heat capacity at constant volume, C,,. The amount of work which must be done in adiabatically compressing an ideal gas in order to elevate the temperature of the gas to a given level can be calculated from the following equations.

where T absolute temperature of compressed gas or hot spot T, absolute temperature of gas bubble V, volume of gas bubble V volume of compressed gas P initial pressure of gas bubble P final pressure of gas bubble C,, heat capacity of gas at constant pressure C heat capacity of gas at constant volume W= work required to accomplish the adiabatic compression Equations (1), (2), and (3) show that as the value of y is increased and all other variables, including T are maintained at a given value, the value of W is decreased. Thus, gases having a relatively high value of 7 require less work input by the high energy shock wave in order to become compressed suffrciently to obtain a given elevated temperature level. For example, helium and argon which have a value of 1.66 for y are preferred to air, nitrogen, hydrogen and oxygen which have a 7 value of 1.4. The former gases are also, of course, inert which adds to their desirability.

A number of difi'erent explosive systems'may be employed in the practice of the invention. It should be pointed out that as the term explosive system is here used, it is intended to encompass systems which undergo ignition as a result of the process, as well as those which explode in the more conventional sense of the word. In any usage of the process, the explosive system acted on will contain at least one component which is either thermally ignited or thermally detonated by reason of proximity to one of the compressed gas hot spots to which reference has been made. Where a thermally ignited component is involved, such component may act as an intermediate in the explosive sequence, thus functioning to detonate or otherwise instigate the primary explosion. The explosive system preferably has a threshold temperature of at least as low as 600 C. above which the described component will be detonated or ignited.

The explosive system is a pumpable semisolid (as a gell, slurry, sol or the like) or a liquid. The pumpability of the explosive system, and its susceptibility to remote ignition or detonation when the invention is employed, permit it to be used in many relatively inaccessible locations with a high degree of precision and safety. Examples of explosive systems which can be used in the practice of the invention include hydrazine and hydrazine nitrate systems incorporated in a suitable oleaginous carrier, sensitized nitroparafi'in systems, and systems containing nitric acid, nitrobenzene and water.

The gas subjected to compression may be disposed in the explosive system in several ways, each of which is intended to randomly locate small, discrete quantities of gas throughout a substantial portion of the system. The gas is preferably present as relatively small bubbles, and may be confined and located by enclosing the gas in small hollow spheres constructed of glass, plastic or gelatinous films. A gelled fluid or an emulsion may also be used to entrap gas bubbles within the explosive mixture. In all cases, the gas is desirably disposed in the explosive system at a pressure which corresponds substantially to the pressure which will act on the system at the situs of the explosion.

The high energy shock wave utilized to compress the gas adiabatically can be generated and propagated in a variety of ways. The energy level of the wave at the locus of the gas undergoing compression can be estimated by the use of Equations (1), (2), and (3), with the realization, of course, that purely adiabatic compression will not be achieved due to some heat transfer between the compressed gas and the surrounding explosive system. An example of a typical calculation of the energy level which must be developed at the gas in order to achieve a hot spot temperature of 600 C. and using helium gas is as follows:

T =298 K. (25 c.)

T =873 K. (600 C.)

V =5 .25X cm (gas sphere radius =5 l0 cm.) Substituting these values in Equations (1), (2), and (3), the work required to achieve the desired hot spot temperature is calculated V 29s 1 2 MiG-1 v, (s73) 5 (1) 1.66 13 87$)F66 1 ,3 P, 29s W e) P V 1 2.11 10 )(5.25 10- 5 1 W:

336 gram cm.

(The negative sig ri ho rs that work done onthe gas bubble.) W

Thus, from the calculations it is seen that in order to raise the temperature of the helium bubbles to 600 C, an energy level of at least 336 gram-cm. must be transmitted to the bubbles by means of the high energy shock wave.

Allowing for some losses of energy in the medium used to couple the source of the wave to the gas-containing explosive system, it will be perceived that several types of shock wave generators may be employed to develop the wave. A low grade explosive can be employed or a high energy acoustical device capable of developing sound waves having the necessary energy content. The shock wave generator is then coupled to the explosive system by a suitable buffer zone coupler. This material is pumpable, substantially incompressible and incompatible with the explosive system. As the term compatible is here used, it is intended to mean that the coupling agent does not mix with and is unreactive with the explosive system or its components. Where the base or carrier material of the explosive system is oleaginous in character, water may be used as the coupling agent. The converse is also true. In pumping the explosive system to the situs of the explosion, it is followed by the coupling agent which extends from a point of contact with the explosive system to the source of the high energy shock wave. The coupling agent is confined so that energy losses to the atmosphere are minimized.

An especially useful application of the present invention is in the stimulation of hydrocarbon production from subterranean formations. In this use of the method of the invention, the productive subterranean formation is traversed by a well bore and then fractured using techniques well understood in the art of petroleum and natural gas production. The explosive system is then made up at the surface to include the small, discrete quantities of gas as hereinbefore described. The gas is placed in the pumpable explosive material at a pressure corresponding to the pressure existing in the fracture, thus assuring that adiabatic compression of the gas bubbles can be effected after the explosive system has been located in the fracture.

The gas-carrying explosive system is then pumped down the well bore and into the fracture. It is followed by the inert buffer zone coupler in sufficient quantity that the coupler material extends from the explosive system into and upwardly in the well bore. If desired, the coupling agent may be extended to the surface and the shock wave generator there actuated, but in mostinstances it will be desirable to lower the shock wave generator device into the well bore into contact with the coupling agent. This permits better confinement and channeling of the high energy shock waves, and reduces the distance which such waves must travel through the coupling agent to reach the explosive system, thus conserving energy. Upon actuation of the shock wave generator, the high energy shock waves are transmitted through the coupling agent to the explosive system located in the fracture. The gas bubbles undergo compression, are heated to a temperature above the ignition or detonation temperature of at least one component of the explosive system, and the end result is an explosion with the fracture. As a result of the explosion, the permeability of the formation is greatly increased, and production is stimulated.

Although the invention has been described primarily with reference to its use for effecting an explosion, and more specifically to its use in stimulating production of fluids from subterranean formations, its usefulness extends to other applications. Thus, it can be used to ignite materials from a remote location in situations where combustion or ignition must be effected in a relatively inaccessible location. Other uses and applications will be suggested by the foregoing description of the invention.

What is claimed is:

l. A method for explosively stimulating production of fluids from a subterranean formation comprising:

preparing a pumpable semisolid or liquid explosive system through which a shock wave can be transmitted having a temperature threshold above which the system will explode;

at a pressure substantially equivalent to the pressure in the subterranean formation, mixing an adiabatically compressible gas with the explosive system to randomly locate discrete quantities of gas in the system;

at the said pressure pumping the gas containing explosive 5 system into the fluid producing subterranean formation;

generating a high energy shock wave at a location spaced from the gas containing explosive system; and

transmitting the high energy shock wave to and through the explosive system to substantially adiabatically compress the gas contained therein.

2. The method defined in claim 1 wherein said gas has a ratio of its heat capacity at constant pressure to its heat capacity at constant volume of 1.66.

3. The method defined in claim 1 wherein said shock wave is generated by:

temperature does not exceed about 600 C.

5. A method for explosively stimulating production of fluids from a subterranean formation comprising:

preparing a pumpable explosive system having a temperature threshold above which the system will explode; mixing helium gas with the explosive system at a pressure substantially equivalent to the pressure in the subterranean formation to randomly locate discrete quantities of helium in the system,

pumping the helium containing explosive system into the fluid producing subterranean formation,

generating a high energy shock wave at a location spaced from the helium containing explosive system by positioning a shock wave generating device in said well bore and actuating said shock wave generating device, and wherein said shock wave is transmitted to and through the expl0- sive system by positioning a substantially incompressible, pumpable coupling agent which is incompatible with said explosive system between said shock wave generating device and said explosive system prior to actuation 0 said shock wave generating device; and

transmitting the high energy shock wave to and through the explosive system to substantially adiabatically compress the gas contained therein.

6. A method for causing the ignition or explosion of a pumpable material comprising:

locating in the material a quantity of gas having a ratio of its heat capacity at constant pressure to its heat capacity at constant volume of 1.66;

generating a high energy shock wave at a location remote from the material; and

transmitting said shock wave from its point of generation through a coupling agent to said material to substantially adiabatically compress said quantity of gas and thereby increase its temperature to above the temperature at which said material will ignite or explode.

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2. The method defined in claim 1 wherein said gas has a ratio of its heat capacity at constant pressure to its heat capacity at constant volume of 1.66.
 3. The method defined in claim 1 wherein said shock wave is generated by: positioning a shock wave generating device in said well bore; and actuating said shock wave generating device; and wherein said shock wave is transmitted to and through the explosive system by positioning a substantially incompressible, pumpable coupling agent which is incompatible with said explosive system between said shock wave generating device and said explosive system prior to actuation of said shock wave generating device.
 4. The method defined in claim 3 wherein said threshold temperature does not exceed about 600* C.
 5. A method for explosively stimulating production of fluids from a subterranean formation comprising: preparing a pumpable explosive system having a temperature threshold above which the system will explode; mixing helium gas with the explosive system at a pressure substantially equivalent to the pressure in the subterranean formation to randomly locate discrete quantities of helium in the system, pumping the helium containing explosive system into the fluid producing subterranean formation, generating a high energy shock wave at a location spaced from the helium containing explosive system by positioning a shock wave generating device in said well bore and actuating said shock wave generating device, and wherein said shock wave is transmitted to and through the explosive system by positioning a substantially incompressible, pumpable coupling agent which is incompatible with said explosive system between said shock wave generating device and said explosive system prior to actuation o said shock wave generating device; and transmitting the high energy shock wave to and through the explosive system to substantially adiabatically compress the gas contained therein.
 6. A method for causing the ignition or explosion of a pumpable material comprising: locating in the material a quantity of gas having a ratio of its heat capacity at constant pressure to its heat capacity at constant volume of 1.66; generating a high energy shock wave at a location remote from the material; and transmitting said shock wave from its point of generation through a coupling agent to said material to substantially adiabatically compress said quantity of gas and thereby increase its temperature to above the temperature at which said material will ignite or explode. 